1. Field of the Invention
The present invention relates to the tunneling magnetoresistive effect element and spin MOS field-effect transistor using a Heusler alloy.
2. Description of the Related Art
Recently, a magnetic memory (magnetic random access memory [MRAM]) using the tunneling magnetoresistive effect (TMR) element (or a magnetic tunnel junction [MTJ] element) having a sandwiched structure including a ferromagnetic material/insulator/ferromagnetic material as a memory element has been proposed. This device is used as a memory by fixing (or maintaining) spins in one ferromagnetic material layer (a fixed layer or reference layer), and controlling (or changing) spins in the other ferromagnetic material layer (a free layer or recording layer), thereby changing the resistance between the two layers in the sandwiched structure. The resistance decreases when the spins in the fixed layer and free layer are parallel, and increases when they are antiparallel. The magnetoresistive change ratio (TMR ratio) as an index of this spin efficiency is a few 10% at room temperature a few years ago, but has reached 500% in recent years. This widens the range of possibility of the device not only as an MRAM but also as various spin devices. As an example, a spin MOS field-effect transistor (spin MOSFET) combined with an MTJ element has been proposed. This makes a double resistance change by the gate electrode and TMR ratio feasible by combining the MTJ element with the spin MOSFET obtained by adding the degree of freedom of spins to carriers.
It is important to increase the TMR ratio in order to realize a high-efficiency magnetic memory or spin MOSFET. To this end, it is necessary to use a ferromagnetic material having a high spin polarization ratio (P). When a semi-metallic material in which P=100% is used, the TMR ratio is theoretically infinite from Julliere's law. Candidates of a room-temperature, semi-metallic material are CrO2, Fe3O4, and a Heusler alloy. Recently, Co-based Heusler alloys have achieved high TMR ratios, so spin devices using these alloys are expected. A Heusler alloy (also called a full-Heusler alloy) is a general term for intermetallic compounds having a chemical composition represented by X2YZ where X is a Co-, Ni-, or Cu-based transition metal element or noble metal element in the periodic table, Y is an Mn-, V-, or Ti-based transition metal, and Z is a main group element of groups III to V. The Heusler alloy X2YZ can be classified into three types of crystal structures in accordance with the regularities of X, Y, and Z. The L21 structure is a structure having the highest regularity, in which X≠Y≠Z, i.e., the three elements can be distinguished from each other. The B2 structure is a structure having the second highest regularity, in which X≠Y=Z. The A2 structure is a structure in which X=Y=Z, i.e., the three elements cannot be distinguished from each other.
To achieve a high TMR ratio by using the Heusler alloy, epitaxial growth of a regular crystal structure is indispensable when forming a stacked structure. The past research reveals that an epitaxially grown B2 structure or L21 structure is necessary to achieve a high spin polarization ratio of the Heusler alloy. Especially when using the Heusler alloy in a spin MOSFET, a technique of forming a highly regular Heusler alloy on a semiconductor is indispensable (e.g., N. Tezuka, et. al., Appl. Phys. Lett. 89(2006)112514).